As a method of manufacturing plastic goods, a method of injection molding is commonly used. In injection molding, in a case where a molding material is a thermoplastic resin such as polyamide (PA), polyphenylene sulphide (PPS), polyether ether ketone (PEEK) or the like, with respect to a plastic heated at a softening temperature (generally 180° C. to 450° C.), an injection pressure is applied and a metal die is filled to form the mold. The molding process is called “mold clamping” since maintaining an inner pressure of the die is necessary until the plastic filling the die has solidified.
In the mold clamping of the injection molding machine in the related art, as a method of applying a mold clamping force, a cylinder drive method using a hydraulic control has mainly been used. However, in recent years, without using a large amount of oil, a ball screw drive method by an electric servo control using a high torque motor which is environmentally friendly and excellent in energy saving performance has been developed and put into practical use.
FIGS. 17 and 18 illustrate an electric injection molding machine 101, which includes a mold clamping unit 102 and an injection unit 103. The mold clamping unit 102 has a rear platen 104, a movable platen 105 and a fixing platen 106, and performs opening and closing of the molding die and the mold clamping through a mold clamping mechanism 108 where a molding die 107 placed between the movable platen 105 and the fixing platen 106 is placed between the rear platen 104 and the movable platen 105.
The mold clamping mechanism 108 has a toggle structure and if a mold clamping ball screw 110 pivotally supported on the rear platen 104 is driven to turn by a mold clamping motor 109, a cross head 111 moving back and forth, is expanded and contracted. As a result, the movable platen 105 is moved back and forth. The mold clamping ball screw 110 is pivotally supported on the rear platen 104 by a support bearing 112.
In addition, the movable platen 105 has a drive mechanism 141 which drives an ejector pin 140 in order to thrust out a product from the molding die 107 after die forming. The drive mechanism 141 has a motor and a ball screw, and rotates an ejector axis 142 forming a screw axis of the ball screw so as to move the ejector pin 140. The ejector axis ball screw is pivotally supported on the movable platen 105 by a support bearing (not illustrated).
The injection unit 103 has a rear plate 113, a movable plate 114 and a front plate 115. A proximal portion of a measurement/injection screw 117 placed in a cylinder assembly 116 of the front plate 115 is pivotally supported on a front surface of the movable plate 114 (mold clamping unit side) via a support bearing 118 (screw sleeve). The measurement/injection screw 117 measures a resin for molding and during melting and kneading, is rotated to turn by a metering motor (not illustrated) attached to the movable plate 114.
As also illustrated in FIG. 5, the movable plate 114 has additionally a ball nut 120 fixed to a rear surface thereof and an injection ball screw 121 screwed thereto is pivotally supported on the rear plate 113, thereby configuring an injection mechanism 122. The injection ball screw 121 is pivotally supported on the rear plate 113 by a support bearing 123 and in addition, is driven by an injection motor 124 attached to the rear plate 113.
If the injection motor 124 is driven in forward direction, the injection ball screw 121 is subject to a positive rotation. Then, the movable plate 114 moves forward and a molten resin inside the cylinder assembly 116 is injected into the molding die 107 by the injection/measurement screw 117. Upon injection, the measurement motor is driven, the injection/measurement screw 117 is rotated and the resin is newly subject to measuring and mixing. At this time, the injection/measurement screw 117 retreats, being pressed by a pressure of the resin fed thereinto. However, a back pressure is exerted by the injection motor 124 such that the mixing and the melting are sufficiently performed. In a stage after completing the measurement, in order to prevent a leakage from a nozzle's tip end, the injection motor 124 is in reverse rotation, the injection/measurement screw 117 is slightly retreated and a sack back is performed.
In addition, the injection unit 103 has a nozzle touch mechanism 153 which brings a ball screw 151 to be in positive rotation or reverse rotation using a drive motor 150 and makes the cylinder assembly 116 proceed back and forth to the fixing platen 106. The nozzle touch mechanism 153 presses a nozzle 116a on a tip end attached to the cylinder assembly 116, using a predetermined force, to the molding die 107 attached to the fixing platen 106. The nozzle touch ball screw 151 is pivotally supported by a support bearing (not illustrated).
Here, in the injection molding machine using an electric servo control, the above described mold clamping ball screw support bearing 112, an ejector axis ball screw support bearing, an injection ball screw support bearing 123 and a nozzle touch ball screw support bearing undertake a large axial load during a rotation support of the ball screw, mold clamping and the like. Accordingly, a specifically dedicated ball screw support bearing using a large diameter ball as illustrated in FIG. 6 comes to be used.
In addition, in a ball screw device disclosed in PTL 1, a multiple row combination ball bearing which supports a screw axis of a ball screw adaptable for the electric injection molding machine is disclosed in that a load carrying capacity of a thrust load is greatly secured by setting a contact angle to be more than or equal to 40 degrees and less than or equal to 65 degrees.